1. Field of the Invention.
The present invention relates to formulating a whisker containing body that can be pressureless sintered to full density in one sintering step or that can be pressureless sintered to closed porosity in preparation for hot isostatic pressing. Generally, this invention pertains to any non-sinterable whisker which is contained in a pressureless-sinterable or sinter-HIP'able and chemically compatible matrix.
2. The State of the Art.
Whiskers are single crystal, high aspect ratio particles which typically have tensile strengths of about 7 GPa and elastic modulus of about 700 GPa.
Sintered silicon carbide whisker reinforced ceramic oxide composites are a relatively new class of engineering materials which have particular applications at high temperatures. The reinforcing whiskers are believed to act as crack blunters and absorb fracture energy, which imparts higher fracture toughness to the brittle ceramic matrix. An analysis of toughening mechanisms, including the possible effect of crack deflection and whisker bridging/pullout, is discussed by J. Homeny et al., in "Processing and Mechanical Properties of SiC-Whisker-Al.sub.2 O.sub.3 -Matrix Composites, " Amer. Cer. Soc. Bull., vol. 66, no. 2, pp. 333-338 (1987). Exemplary of the possible application benefits, Tiegs and Becher (J. Amer. Cer. Soc., vol. 2, p. 339 (1986)) have shown that the presence of even low (i.e., &lt;10 vol. %) SiC whisker contents improves various properties, including thermal shock resistance, toughness, and high temperature strength. Thus, proper engineering of these materials should allow the structural ceramic component designer to achieve key property improvements.
The variables which influence the sinterability of a ceramic powder compact can be separated into two groups: powder characteristics and arrangement characteristics. Powder characteristics include particle size (both average and the distribution of particle sizes), particle shape (e.g., sphericity, aspect ratio), surface chemistry (e.g., isoelectric point), and the overall powder composition (e.g., mixtures of different particles). Arrangement characteristics are dependent upon the powder characteristics and the dynamics of particle ordering during processing, forming, and consolidation; arrangement characteristics include green density, green density distribution, pore size, and pore size distribution.
Fabrication of ceramic-whisker composites has not been straightforward; elaborate schemes have been devised to attain full density of the composite matrix upon sintering. Whiskers, which are non-sinterable and inert, act as proppants which retard the densification of the matrix particles. Traditionally, hot pressing or sinter-HIP with sintering aids have been used to densify these composites. Hot Pressing (HP) involves uniaxially pressing a green ceramic compact under high temperature and pressure; this process is limited to simple shapes that can be uniaxially formed, such as disks and billets. The green ceramic compacts can have open porosity prior to sintering, and the compacts can have up to 50% total porosity. The Hot Isostatic Pressing (HIP) process is where a green compact is formed, of either a simple or a complex shape, and the compact is subsequently sintered in an isostatic stressfield at high temperature and pressure (e.g., &lt;20,000 psi). In this case, all of the porosity present in the green compact must be sealed off at the surface, i.e., all of the porosity must be "closed"; this can be accomplished either by first sintering the piece to closed porosity, typically .ltoreq.10 vol. % porosity, or by encapsulating the compact in glass prior to HIP'ing. The technique generally preferred in the art is the sinter-HIP densification scheme, in which one must have the ability to pressureless sinter the ceramic to a density of approximately .gtoreq.90-92% of theoretical density to effectively close the porosity. Further, because the second step (pressure sintering, HP, or HIP) can cause ceramic degradation at the expense of densification, and HP or HIP is a costly batch step requiring expensive process equipment and very careful sample preparation, the achievement of pressureless sintered densities approaching theoretical density for high whisker loadings would be very beneficial. Still further, pressure sintering typically results in an overfired ceramic matrix, often leading to degradation and formation of a glassy phase at the expense of full densification; accordingly, providing pressureless sintered samples of a high density would help to reduce the degradation caused in the pressure sintering step. Also, a typical hot pressed SiC whisker/alumina composite at full density will exhibit alumina grains in excess of 50 microns, an order of magnitude larger than the original alumina particles, thereby resulting in strength limiting defects in the stressed body.
As a reinforcing component in the sintered ceramic matrix, Wei (U.S. Pat. No. 4,543,345) has shown that pressure sintered composites of silicon carbide whiskers and a fine ceramic powder (such as alumina, mullite, or boron carbide) provide a significant fracture toughening component to the brittle ceramic microstructure. Wei prepares the composites by sintering at 1600.degree. C. to 1950.degree. C. at pressures of 28 to 70 MPa.
Tiegs (U.S. Pat. No. 4,652,413) augmented Wei's composition by the addition of 0.5 to 5 wt. % of a sintering aid such as yttria to a body containing 5 to 10 vol. % silicon carbide whiskers, and the balance being 0.1 to 1.0 micron alumina. Tiegs begins with a granulated mixture of alumina, whiskers, and sintering aid, which is cold pressed into a pellet having a density of approximately 55% theoretical. The pellet is then two-step sintered to full density by first pressureless sintered at 1800.degree. C. to 95% of theoretical density, followed by a pressure sintering step at 20,000 psi to over 98% of theoretical density. Tiegs also states that pellets with 20 vol. % whiskers could not be sintered to over 75% theoretical density, although increased yttria content may be beneficial. Moreover, Tiegs also states that pellets with whisker concentrations exceeding 10 vol. % may not be sinterable without pressure assistance.
Becher and Tiegs (in U.S. Pat. No. 4,657,877) also built upon Wei's work by the addition of transformation toughened tetragonal zirconia to the SiC whisker-mullite and SiC whisker-alumina matrix. The further addition of ZrO.sub.2 to the whisker composite and sintering at 7 to 70 MPa in an inert atmosphere provided further increased toughness.
Thus, the art has continuously resorted to fabrication methods for whisker reinforced composites which require a pressure sintering step. The art has also suggested, in certain aspects, that pressure sintering and/or an increased amount of sintering aid is necessary for compositions having greater than a threshold volume fraction of whiskers. However, the use of a pressure sintering step can limit the complexity and geometry of the composite articles fabricated, as well as adding significantly to the processing costs due to the complexity of the articles.
It is also important to note that SiC whiskers commercially available vary dramatically in their properties. Tiegs and Becher (ORNL/TM-9947, 1985-86) reviewed SiC whisker reinforced ceramic composites, and thereby discovered significant differences among whiskers. Among various whiskers (i.e., those available from ARCO, Tokai Carbon, Tateho, Versar, and Los Alamos National Laboratory) incorporated at 20 vol. % into an alumina matrix, sintered densities ranged from 3.72 to 3.83 g/cm.sup.3, strengths varied from 340 to 650 MPa, fracture toughnesses varied from 4.2 to 9.1 MPa .sqroot.m, and weight losses during hot pressing varied from 0.8% to 5.17%. In some instances (with Tateho and Tokai Carbon whiskers), while the whiskers appeared to be intact, SEM examination of a fracture surface revealed no SiC whiskers, believed to be due to adhesive interaction between the matrix and whisker phases, and thus no benefits of pullout or crack deflection. Tiegs, Becher, and Harris (ORNL/TM-9947, 1984-85) have also shown that for 20 vol. % SiC reinforced alumina including Tateho and ARCO whiskers, the respective fracture toughnesses were 5.1 and 7.8, and the respective flexural strengths were 535 and 700. The 1984-85 report also showed the good thermal shock resistance of 20 vol. % SiC-alumina composites up to about 900.degree. C. Thus, SiC whiskers can provide desired properties, but the properties vary widely among whiskers, which thereby limits the number of variables for which a certain ceramic can be engineered. See also T. Tiegs, "Business Outlook for Advanced Ceramics and Composites," Whisker-Reinforced Ceramic Composites: Present Status and Potential Trends, presentation at Oak Ridge National Laboratory on Mar. 12, 1987.
As noted above, pressuring sintering processes typically degrade the surface layer of the oxide ceramic matrix, usually by oxidizing the same, often necessitating machining of the surfaces prior to use. Accordingly, it would be beneficial to decrease or eliminate this degradation and the subsequent machining; generally, it would be necessary to modify the matrix such that it sinters faster and at a lower temperature to achieve such benefits.